Various products and systems, including consumer products such as TVs, electric powered vehicles, radar systems, electric motor controllers and uninterrupted power supply (UPS) systems, require provision of relatively large amounts of electric power often provided from a high voltage power supply. Various types of semiconductor field effect transistors (FETs) based on silicon materials and technology are generally used as power switches to perform switching functions required by the products and systems.
A FET typically comprises terminals referred to as a “source” and a “drain” for connecting a power source to a load and a terminal in the FET located between the source and drain referred to as a “gate” for controlling resistance of a current carrying channel in the FET located under the gate between the source and drain. Voltage relative to a common ground voltage applied to the gate generates an electric field in the FET that controls the resistance of the channel to turn the transistor ON and OFF. When turned ON, voltage applied the gate reduces the resistance of the channel to allow relatively large current flow between the source and drain. Total resistance between the source and drain when the transistor is turned ON is referred to as an “ON resistance” of the transistor. The ON resistance depends upon the resistance of the channel, resistance to current flow of a region of the FET under, and in the neighborhood, of the source, and resistance of a region of the FET under, and in the neighborhood, of the drain. The regions under and in the neighborhoods of the source and drain are conventionally referred to as access regions.
Whereas conventional power FETs based on Si provide useful switching functions, they are not readily configured to provide desired characteristics for power switching applications for, by way of example, operation of devices such as, electric motors and vehicles, uninterruptable power supplies (UPS) and photovoltaic inverters. Switches suitable for operation of these devices are advantageously characterized by relatively high breakdown voltage when they are OFF, high “ON currents” between source and drain when they are ON, and relatively low gate and drain leakage currents. It is advantageous that they are capable of operating at high junction temperatures and that they exhibit good tolerance to current and/or voltage transients that tend to occur during switching between OFF and ON states. In addition, for safety reasons, preferably the switches are OFF when their gates are at ground potential.
For example, it can be advantageous for a semiconductor power switch to have a breakdown voltage equal to or greater than about 600 V and drain leakage currents less than about 100 μA per mm (millimeter) of gate periphery when OFF. When ON it is advantageous that the switch, have an ON resistance less than or about equal to 10 Ohm per mm and be capable of safely supporting a drain current greater than or equal to about 50 A (amps). In addition, for safety reasons, it is generally advantageous that the switch be OFF for gate voltages less than about 2 volts, and be able to operate without damage to itself at junction temperatures greater than or equal to about 200° C. Semiconductor switches based on Si materials and technology are generally not readily configurable to provide these specifications because their band gaps, which are typically less than about 2 eV (electron volts), and saturation drift velocities of electrons in the materials do not naturally support high breakdown voltages and large ON current.
Nitride based semiconductor materials, such as GaN (Gallium Nitride) and AlN (Aluminum Nitiride) on the other hand, are characterized by relatively large band gaps of 3.4 eV and 6.2 eV respectively. And FETs having a nitride semiconductor layer structure comprising a small band gap layer adjacent a large band gap layer provide a relatively high concentration of high mobility electrons characterized by a high saturation drift velocity. The high mobility electrons accumulate in a narrow triangular potential well at an interface between the layers to form a relatively thin, sheet-like electron concentration, referred to as two dimensional electron gas (2DEG). Because of the geometrical construction and location of the 2DEG, electrons in the 2DEG generally evidence very low donor impurity scattering, and as a result, the relatively high electron mobility, which may for example be equal to about 1.5×107 cm/s. Concentrations of electrons in a 2DEG may be as high as 1×1013/cm2.
FET transistors that operate by generating and controlling high mobility electrons in 2DEGs are conventionally referred to as high electron mobility transistors “HEMTs”. Semiconductor layer structures comprising layers of different composition that characterize these transistors are referred to as “heterostructures”, and interfaces between two adjacent layers of different composition are referred to as “heterojunctions”.
Whereas the inherent characteristics of nitride based semiconductor materials appear to make them excellent materials for use in producing high power semiconductor switches, it has proven difficult to exploit the characteristics to provide such switches. For example, 2DEG nitride FETs are normally ON, rather than being the desired, normally OFF, and it has been found difficult to produce nitride semiconductor layers having defect concentrations sufficiently low to produce power FETs having desired characteristics at acceptable costs.